(1) Field of the Invention
The present invention relates to a magnetic core for a saturable reactor and others to be incorporated in a magnetically controllable switching mode of power source and others where the magnetic core is used to control its out-put voltage. More particularly, it relates to a magnetic core having excellent magnetization characteristics for the control during its operation in a high frequency of alternating-current(AC) magnetic field, a high rectangular ratio(Br/Bs), a low core loss(W) and a low disaccommodation of its magnetic characteristics.
(2) Description of the Prior Art
Because of recent development of electronic devices, switching power sources carrying a magnetic amplifier have been getting more and more widely used. A main portion constituting a magnetic amplifier is a saturable reactor comprising a magnetic core including a metal ribbon wound into a toroidal shape. Magnetic core materials provided with a high saturation flux density Bs and a high rectangular ratio Br/Bs where Br represents a residual magnetic flux density are now desired to use for cores of the saturable reactors in switching power sources. Heretofore, there have been used 50% Ni-Permalloys, 80% Ni-Permalloys, etc. But they fail to meet the recent high frequency requirements of switching power sources which have increasingly high performance with reduced size and weight. Specifically, 50% Ni-Permalloys and 80% Ni-Permalloys suffer enormous core loss when they are used in a high frequency magnetic field. So, magnetic core materials excellent in high frequency characteristics have been required.
A proposal has been made to provide a magnetic core comprising a heat-treated Co-base amorphous alloy ribbon wound into a toroidal shape to overcome the abovestated disadvantages of 50% Ni-Permalloys and 80% Ni-Permalloys as described in U.S. Pat. No. 4,473,417. One method of heat treatments applied to Co-base amorphous alloy is to quench it after keeping it at a temperature higher than its Curie temperature(Tc).
Although amorphous alloys processed by the above heat treatment may have a low initial core loss, they suffer a low rectangular ratio(Br/Bs), a low magnetic anisotropy, and a big disaccommodation of core loss. At a result, saturable reactors made of such materials have a tendency to become uncontrollable because of its excessive heat generation, also causing undesirable effects on the neighboring elements in a switching power source.
It was also proposed to treat a Co-base amorphous alloy ribbon at a temperature below its Curie temperature where the alloy remains magnetic, from the aspect that a Co-base amorphous alloy is easily provided with an inductive magnetic anisotropy, because an amorphous state is a metastable state metallurgically. In the above-mentioned case, an amorphous metal alloy ribbon having a low core loss is produced by a rapid quench of a melt, then heat-treated at a temperature below its Curie temperature in a magnetic field in order to enhance its rectangular ratio Br/Bs.
However, it was recently recognized that a toroidal core comprised of an amorphous alloy which was heat-treated by the above-mentioned method has a big rectangular ratio and a small disaccommodation of core loss, but the core loss thereof is bigger than those of amorphous alloy heat-treated at a temperature above its Curie temperature.
The other proposals to improve its magnetic characteristics have been made to provide a magnetic core comprising a heat-treated Co-base amorphous alloy ribbon wound into a toroidal shape in Japan Patent Publications Nos. 60-19125, 58-1183 and 59-29644. Amorphous metal ribbons having a composition represented by a formula: (Fe, Ni, Co).sub.90-70 (Si,B, P, C).sub.30-10 in Japan Patent Publication No. 60-19125 and a formula: (Fe, Ni).sub.0.1-40 Si.sub.3-16 B.sub.5-24 in Japan Patent Publication No. 58-1183 are subjected to a heat-treatment to cool them in a magnetic field or to cool them rapidly in an air. In Japan Patent Publication No. 59-29644, a heat-treatment in a magnetic-field for an amorphous metal alloy is disclosed. The aim of the treatment is to reduce a coercive force in a direct-current(DC) B-H curve of an amorphous metal alloy, where the amorphous metal alloy is cooled at a cooling rate in a predetermined temperature range. However it is more important to reduce a coercive force in an alternating-current(AC) B-H curve of an amorphous metal alloy from a practical point of view. It is also important to reduce an iron loss of an amorphous metal alloy and to provide it with a high rectangular ratio(Br/Bs) where Br is a residual magnetic flux density and Bs is a saturation flux density, measured in a high frequency magnetic field of 50 KHz or more.